[DRIVERS] i.MX: Add full serial driver for i.MX.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
[ARM] i.MX31: fix i.MX31 dtsi file and add uart3

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>